Chemical heat source comprising metal nitride, metal oxide and carbon

ABSTRACT

A chemical heat source comprising metal nitride, metal oxide and carbon, particularly useful in smoking articles, and methods of making the heat source are provided. The metal nitride of the heat source has an ignition temperature substantially lower than conventional carbonaceous heat sources, while at the same time provides sufficient heat to release a flavored aerosol from a flavor bed for inhalation by the smoker. Upon combustion the heat source produces virtually no carbon monoxide. The metal nitride is prepared by pre-forming the starting materials into a desired shape, and converting them to metal nitride in situ, without substantially altering the shape of the starting materials.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of commonly-assigned U.S.patent application Ser. No. 7/443,636 (PM-1389), filed Nov. 29, 1989,U.S. Pat. No. 5,188,130, which is hereby incorporated by reference inits entirety.

BACKGROUND OF THE INVENTION

This invention relates to a metal nitride heat source and to improvedmethods for making the heat source. The methods and heat sources of thisinvention are particularly suitable for use in a smoking article, suchas that described in Serrano et al. U.S. Pat. No. 4,991,606 (PM-1322)and Serrano et al. U.S. Pat. No. 4,966,171 (PM-1322CIP), which are bothhereby incorporated by reference in their entireties. The heat sourceshave low ignition and high combustion temperatures and generatesufficient heat to release a flavored aerosol from a flavor bed forinhalation by the smoker. Upon combustion, the heat sources of thisinvention produce substantially no carbon monoxide or nitrogen oxides.

According to the methods of this invention, the metal species is mixedwith a carbon source, heated and converted to metal nitride bycontacting the mixture with a nitriding material. In a preferredembodiment, the metal species/carbon source mixture is pre-formed into adesired shape and converted to metal nitride in Situ, withoutsubstantially altering the shape of the mixture.

There have been previous attempts to provide a heat source for a smokingarticle. While providing a heat source, these attempts have not produceda heat source having all of the advantages of the present invention.

For example, Siegel U.S. Pat. No. 2,907,686 discloses a charcoal rodcoated with a concentrated sugar solution which forms an imperviouslayer during burning. It was thought that this layer would contain gasesformed during smoking and concentrate the heat thus formed.

Ellis et al. U.S. Pat. No. 3,258,015 and Ellis et al. U.S. Pat. No.3,356,094 disclose a smoking device comprising a nicotine source and atobacco heat source.

Boyd et al. U.S. Pat. No. 3,943,941 discloses a tobacco substitute whichconsists of a fuel and at least one volatile substance impregnating thefuel. The fuel consists essentially of combustible, flexible andself-coherent fibers made of a carbonaceous materials containing atleast 80% carbon by weight. The carbon is the product of the controlledpyrolysis of a cellulose-based fiber containing only carbon, hydrogenand oxygen.

Bolt et al. U.S. Pat. No. 4,340,072 discloses an annular fuel rodextruded or molded from tobacco, a tobacco substitute, a mixture oftobacco substitute and carbon, other combustible materials such as woodpulp, straw and heat-treated cellulose or a sodiumcarboxymethylcellulose (SCMC) and carbon mixture.

Shelar et al. U.S. Pat. No. 4,708,151 discloses a pipe with replaceablecartridge having a carbonaceous fuel source. The fuel source comprisesat least 60-70% carbon, and most preferably 80% or more carbon, and ismade by pyrolysis or carbonization of cellulosic materials such as wood,cotton, rayon, tobacco, coconut, paper and the like.

Banerjee et al. U.S. Pat. No. 4,714,082 discloses a combustible fuelelement having a density greater than 0.5 g/cc. The fuel elementconsists of comminuted or reconstituted tobacco and/or a tobaccosubstitute, and preferably contains 20%-40% by weight of carbon.

Published European patent application 0 117 355 by Hearn et al.discloses a carbon heat source formed from pyrolized tobacco or othercarbonaceous material such as peanut shells, coffee bean shells, paper,cardboard, bamboo, or oak leaves.

Published European patent application 0 236 992 by Farrier et al.discloses a carbon fuel element and process for producing the carbonfuel element. The carbon fuel element contains carbon powder, a binderand other additional ingredients, and consists of between 60 and 70% byweight of carbon.

Published European patent application 0 245 732 by White et al.discloses a dual burn rate carbonaceous fuel element which utilizes afast burning segment and a slow burning segment containing carbonmaterials of varying density.

These heat sources are deficient because they provide unsatisfactoryheat transfer to the flavor bed, resulting in an unsatisfactory smokingarticle, i.e., one which fails to simulate the flavor, feel and numberof puffs of a conventional cigarette.

Nystrom et al. U.S. Pat. No. 5,076,296, which is hereby incorporated byreference in its entirety, solved this problem by providing acarbonaceous heat source formed from charcoal that maximizes heattransfer to the flavor bed, releasing a flavored aerosol from the flavorbed for inhalation by the smoker, while minimizing the amount of carbonmonoxide produced.

However, all conventional carbonaceous heat sources liberate some amountof carbon monoxide gas upon ignition. Moreover, the carbon contained inthese heat sources has a relatively high ignition temperature, makingignition of conventional carbonaceous heat sources difficult undernormal lighting conditions for a conventional cigarette.

Attempts have been made to produce non-combustible heat sources forsmoking articles in which heat is generated electrically. E.g., Burruss,Jr., U.S. Pat. No. 4,303,083, Burruss U.S. Pat. No. 4,141,369, GilbertU.S. Pat. No. 3,200,819, McCormick U.S. Pat. No. 2,104,266 and Wyss etal. U.S. Pat. No. 1,771,366. These devices are impractical and none hasmet with any commercial success.

Attempts have been made to produce a combustible, non-carbonaceous heatsource. Copending U.S. patent application Ser. No. 281,496, filed onDec. 12, 1988 (PM-1326) and commonly assigned herewith relates the useof a metal carbide heat source. Although combustion of the metal carbideheat source yields up to tenfold less carbon monoxide than combustion ofconventional carbonaceous heat sources, some carbon monoxide is stillproduced.

Attempts have been made to produce pyrophoric materials comprising metalaluminides for use as a decoy for heat-seeking missiles. E.g., Baldi,U.S. Pat. No. 4,799,979. These devices, however, combust too rapidly andproduce too intense a heat to be used as a heat source in a smokingarticle.

Methods of producing iron nitride by converting iron oxide to ironnitride are known. These methods generally involve treating metalliciron with a hydrogen/ammonia/molecular nitrogen mixture. E.g., K. H.Jack, Proceedings of the Royal Society, A, 195 pp. 34-40 (1948); K. H.Jack, Acta Crystallographica, 5, pp. 404-411 (1952); K. H. Jack,Proceedings of the Royal Society, A, 208, pp. 200-215 (1952); KnauffU.S. Pat. No. 3,681,018. These methods produced the iron nitride by aseries of non-continuous steps, rendering these methods unsuitable forthe large scale production of iron nitride. Moreover, the iron nitrideproduced by these methods possesses high thermal stability and lowchemical reactivity and is, therefore, difficult to ignite, rendering itunsuitable for use as a heat source.

A further shortcoming of known methods of preparing iron nitride is thatiron nitride is produced only in particulate form and must be formedinto a shape suitable for use as a heat source. Iron nitrides by natureare brittle, intractable materials, which, once formed, are difficultand expensive to form into a suitable shape.

It would be desirable to provide a heat source that liberates virtuallyno carbon monoxide or nitrogen oxides upon combustion.

It would also be desirable to provide a heat source that has a lowtemperature of ignition to allow for easy lighting under conditionstypical for a conventional cigarette, while at the same time having acombustion temperature high enough to provide sufficient heat to releaseflavors from a flavor bed.

It would further be desirable to provide a heat source that does notself-extinguish prematurely.

It would also be desirable to provide a heat source which is stable atambient temperature and humidity.

It would be desirable to provide an inexpensive method of producingmetal nitride which allows for control of end product distribution.

It would further be desirable to provide a method of producing metalnitride in which the starting materials are pre-formed into a desiredshape and converted in situ to metal nitride.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a heat source thatliberates virtually no carbon monoxide or nitrogen oxides uponcombustion.

It is also an object of this invention to provide a heat source that hasa low ignition temperature to allow for easy lighting under conditionstypical for a conventional cigarette, while at the same time having acombustion temperature high enough to provide sufficient heat to releaseflavors from a flavor bed.

It is yet another object of this invention to provide a heat source thatdoes not self-extinguish prematurely.

It is yet another object of this invention to provide heat source whichis stable at ambient temperature and humidity.

It is a further object of this invention to provide an inexpensivemethod of producing metal nitride which allows for control over endproduct distribution.

It is also an object of this invention to provide a method of producingmetal nitride in which the starting materials are pre-formed into adesired shape and converted in situ to metal nitride.

In accordance with this invention, the heat source is formed frommaterials having a substantial metal nitride content. Preferably, theheat source comprises substantially metal nitride, with smaller amountsof carbon, and metal oxide. Catalysts and burn additives may be added topromote complete combustion and to provide other desired burncharacteristics.

Metal nitrides are hard, brittle compounds characterized by high meltingpoints. Metal nitrides are interstitial alloys having atomic nitrogenbound in the interstices of the parent metal lattice. The nitridelattice is closely related to the cubic or hexagonal close-packed puremetal lattice. Metal nitrides can have a wide range of stoichiometries.Iron nitride, for example, can have formulas ranging from Fe₂ N to Fe₁₆N₂ (Goldschmidt, H. I. Interstitial Alloys, pp. 214-231 (Butterworth,London, 1967)).

Upon combustion, the metal nitride heat sources of this inventionliberate substantially no carbon monoxide or nitrogen oxides. The metalnitride has an ignition temperature substantially lower than that ofconventional carbonaceous heat sources, and is, therefore, easier tolight. Once ignited, the carbon component of the heat source yieldsadditional heat upon combustion, thereby preventing prematureself-extinguishment. While not wishing to be bound by theory, it isbelieved that combustion of the metal nitrides produces metal oxides andmolecular nitrogen, without formation of any significant amount ofnitrogen oxides. The metal oxides act as oxidation catalysts to promotethe conversion of carbon monoxide (CO) to carbon dioxide (CO₂).

According to the method of this invention, a metal species and a carbonsource are combined and heated to reduce the metal species. Preferablythe metal species/carbon source is pre-formed into a desired shapebefore heating. The mixture is then contacted with a nitriding materialwhich reacts preferentially with the reduced metal. The product of thenitridation reaction is a heat source comprising metal nitride, metaloxide, and carbon, which has retained the shape of the metalspecies/carbon source starting materials.

While the heat sources of this invention are particularly useful insmoking devices, it is to be understood that they are also useful asheat sources for other applications, where having the characteristicsdescribed herein is desired.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of this invention will beapparent upon consideration of the following detailed description, takenin conjunction with the accompanying drawings, in which like referencecharacters refer to like parts through out, and in which:

FIG. 1 depicts an end view of one embodiment of the heat source of thisinvention;

FIG. 2 depicts a longitudinal cross-sectional view of a smoking articlein which the heat source of this invention may be used; and

FIG. 3 depicts heat vs. reaction time for the chemical conversion of theiron species to iron nitride. The origin in FIG. 3 is the point at whichheat is supplied to the iron species/carbon mixture.

DETAILED DESCRIPTION OF THE INVENTION

Smoking article 10 consists of an active element 11, an expansionchamber tube 12, and a mouthpiece element 13, overwrapped by a cigarettewrapping paper 14. Active element 11 includes a metal nitride heatsource 20 and a flavor bed 21 which releases flavored vapors whencontacted by hot gases flowing through heat source 20. The vapors passinto expansion chamber tube 12, forming an aerosol that passes tomouthpiece element 13, and then into the mouth of a smoker.

Heat source 20 should meet a number of requirements in order for smokingarticle 10 to perform satisfactorily. It should be small enough to fitinside smoking article 10 and still burn hot enough to ensure that thegases flowing therethrough are heated sufficiently to release enoughflavor from flavor bed 21 to provide flavor to the smoker. Heat source20 should also be capable of burning with a limited amount of air untilthe metal combusting in the heat source is expended. Upon combustion,heat source 20 should produce substantially no carbon monoxide or any ofthe nitrogen oxides. Combustion, the interaction of the heat source withoxygen during puffing to produce neat and light, is flameless andglowing.

Heat source 20 should have a surface area preferably in the range fromabout 65 m² /g to about 150 m² /g, more preferably from about 115 m² /gto about 130 m² /g. Additionally, the heat source made by this inventioncontain macropore (pores of between about 1 and about 5 microns),mesopores (pores of between about 20 Å and about 500 Å in size) andmicropores (pores of up to about 20 Å in size).

Heat source 20 should have an appropriate thermal conductivity. If toomuch heat is conducted away from the burning zone to other parts of theheat source, combustion at that point will cease when the temperaturedrops below the extinguishment temperature of the heat source, resultingin a smoking article which is difficult to light and which, afterlighting, is subject to premature self-extinguishment. Suchself-extinguishment is also prevented by having a heat source thatundergoes essentially 100% combustion. The thermal conductivity shouldbe at a level that allows heat source 20, upon combustion, to transferheat to the air flowing through it without conducting heat to mountingstructure 24. Oxygen coming into contact with the burning heat sourcewill almost completely oxidize the heat source, limiting oxygen releaseback into expansion chamber tube 12. Mounting structure 24 should retardoxygen from reaching the rear portion of the heat source 20, therebyhelping to extinguish the heat source after the flavor bed has beenconsumed. This also prevents the heat source from falling out of the endof the smoking article.

The metal nitride used to make the heat sources of this invention may beprepared by:

1) mixing a metal species with a carbon source to form a mixture;

2) supplying heat to the mixture; and

3) contacting the mixture with a nitriding material to form a metalnitride.

The metal species may be any metal-containing molecule capable of beingconverted to metal nitride. Preferably, the metal species is zirconiumoxide, iron oxide, metallic zirconium, metallic iron, or a mixturethereof. More preferably, the metal species is iron oxyhydroxide, Fe₃O₄, or FeO, and, most preferably, Fe₂ O₃. Different phases of thevarious metal species may be used without substantially affecting themethod of the invention or the course of the nitridation reaction.Either a naturally-occurring or synthetic metal species may be used.

The carbon source is added in the form of substantially pure carbon,although materials which may be subsequently converted to carbon may bealso used. Preferably, the carbon source is colloidal graphite, and,more preferably, activated carbon or activated charcoal.

In combining the metal species with a carbon source, a sufficient amountof carbon should be added to the metal species so that some carbonremains in the composition following the nitridation step. Preferably,between about 5% and about 45% and, more preferably, between about 15%and about 35% by weight of carbon is added to form the metalspecies/carbon source mixture.

The metal species/carbon source should be in particulate form.Preferably, the particle size of the metal species and carbon sourcerange up to about 300 microns. More preferably, the particles of themetal species should range in size between about submicron and 20microns, while the particle size of the carbon source should range insize between about submicron and about 40 microns. The particles may beprepared at the desired size, or they may be prepared at a larger sizeand ground down to the appropriate size.

The surface areas of the metal species and the carbon source particlesare critical. The greater the surface area, the greater the reactivityof the metal species and the carbon source, resulting in a moreefficient conversion. Preferably, the surface area of the metal speciesparticles should range between about 0.2 m² /g to about 300 m² /g. Morepreferably, the particles should have a surface area of between about 1m² /g and about 150 m² /g. Preferably, the carbon particles should rangein surface area between about 0.5 m² /g and about 2000 m² /g. Morepreferably, the carbon particle surface area should range between about100 m² /g and about 600 m² /g.

The metal species and the carbon source may be combined in a solvent.Any solvent which increases the fluidity of the metal species/carbonsource mixture and does not affect the chemical reactivities of theindividual components may be used. Preferred solvents include polarsolvents such as methanol, ethanol, and acetone and, most preferably,water.

The metal species/carbon source mixture may then be combined with acarbonaceous binder which confers greater mechanical stability to themetal species/carbon source mixture. During the conversion of themixture to metal nitride, the binder decomposes into carbon, carbondioxide and carbon monoxide. The metal species/carbon source mixture maybe combined with the binder using any convenient method known in theart.

Any number of binders can be used to bind the particles of the metalspecies/carbon source mixture. The binder material may be used incombination with other additives such as potassium citrate, sodiumchloride, vermiculite, bentonite, calcium carbonate, flavorants, or burnadditives. Any binder that decomposes upon heating to CO and CO₂ may beused. Preferable binders are carbonacous, and include gums such as guargum, cellulose derivatives, such as methylcellulose,carboxymethylcellulose, and hydroxypropyl cellulose, flour, starches,sugar, alginates, polyvinyl alcohols, and vegetable oil, or mixturesthereof. An especially preferred carbonaceous binder material is amixture of flour and sugar combined with corn oil, preferably at a ratioof about 200 parts flour, about 103 parts sugar and about 26 parts cornoil. The metal oxide/carbon mixture as described above is preferablycombined with the flour/sugar/corn oil binder system along with asolvent so that the mixture has a consistency suitable for extrusion.

The metal species/carbon source mixture may then be pre-formed into adesired shape. Any method capable of pre-forming the mixture into adesired shape may be used. Preferred methods include slip casting,injection molding, and die compaction, and, most preferably, extrusion.

Any desired shape may be used in the method of this invention. Thoseskilled in the art will understand that a particular application mayrequire a particular shape. In a preferred embodiment, the mixture isformed into an elongated rod. Preferably, the rod is about 30 cm inlength. The diameter of the heat source may range from about 3.0 mm toabout 8.0 mm, preferably between about 4.0 mm to about 5.0 mm. A finaldiameter of approximately 4.0 mm allows an annular air space around theheat source without causing the diameter of the smoking article to belarger than that of a conventional cigarette. The rods before baking arecalled green rods. Because variations in the dimensions of the rod mayoccur during baking (see discussion, infra), it is preferable to formthe green rods at a slightly larger diameter than the final diameter ofthe heat source.

In order to maximize the transfer of heat from the heat source to flavorbed 21, one or more air flow passageways 22, as described in copendingU.S. patent application Ser. No. 223,232, may be formed through or alongthe circumference of heat source 20. The air flow passageways shouldhave a large geometric surface area to improve the heat transfer to theair flowing through the heat source. The shape and number of thepassageways should be chosen to maximize the internal geometric surfacearea of heat source 20. Alternatively, the heat source may be formedwith a porosity sufficient to allow heat flow through the heat source.When the heat source is ignited and air is drawn through the smokingarticle, the air is heated as it passes around or through the heatsource or through, over or around the air flow passageways. The heatedair flows through a flavor bed, releasing a flavored aerosol forinhalation by the smoker. Any configuration that gives rise to asufficient number of puffs and minimizes the CO produced either underFTC conditions or under more extreme conditions that a smoker may createis within the scope of this invention. Preferably, when longitudinal airflow passageways such as those depicted in FIG. 1 are used, maximizationof heat transfer to the flavor bed is accomplished by forming eachlongitudinal air flow passageway 22 in the shape of a multi-pointedstar. Even more preferably, as set forth in FIG. 1, each multi-pointedstar should have long narrow points and a small inside circumferencedefined by the innermost edges of the star. These star-shapedlongitudinal air flow passageways provide a larger area of heat source20 available for combustion, resulting in a greater volume ofcomposition involved in combustion, and, therefore, a hotter burningheat source.

The green rods are then placed on graphite sheets which are stacked oneover the other in a stainless steel container or on a stainless steelframe. The container containing the stacked graphite sheets is thenplaced in a heating or baking device such as a muffle furnace or asagger. Once inside the heating device, the rods are exposed to anenvironment that will allow the conversion of the metal species to metalnitride. Preferably, the heating device is pressurized slightly aboveone atmosphere to prevent diffusion of gases from the externalatmosphere to within the heating device.

The chemical conversion may be accomplished by supplying heat to thegreen rods. Heat may be supplied in a variety of ways as follows: 1) sothat a constant temperature is maintained; 2) in a series of intervals;3) at an increasing rate, which may be either constant or variable; or4) combinations therof. Additionally, steps such as allowing the rods tocool may be employed. Preferably, however, heat is supplied, asdescribed in FIG. 3, in a multiple stage baking process involving binderburnout followed by nitridation. Those skilled in the art willunderstand that thermal processes (such as solvent vaporization andbinder burnout) may occur at a wide variety of temperatures andpressures.

Binder burnout involves the vaporization of any solvent present in therod as well as the vaporization and carbonization of the carbonaceousbinder. Furthermore, some reduction of the metal species occurs. Binderburnout is accomplished by gradually supplying heat to the rod under aninert atmosphere such as helium, nitrogen, or argon, or in a vacuum. Itis preferable to supply heat to the rod at a first, low rate ofincrease, followed by a second, greater rate of increase.

The first low rate of temperature increase allows for vaporization ofany solvent present in the rod without formation of ruptures and cracksin the rod. Additionally, a low rate of temperature increase minimizeswarping and bending of the rod. The initial rate of increase should bebetween about 0.1° C./min to about 10° C./min, and preferably in therange of about 0.2° C./min to about 5° C./min. This rate of increase ismaintained until a temperature in the range of about 100° C. to about200° C., and a more preferable temperature is about 125° C., is reachedand all solvents are vaporized.

Once the solvent in the rod has been vaporized, the rate of heating isincreased to further decompose carbonaceous binders in the rod and toreduce the metal species. The carbonaceous binder begins to decompose attemperatures in the range of about 200° C. to about 300° C. to a gaseousmixture comprising carbon monoxide and carbon dioxide. Consequently, therate of heating should be such that the evolution of gaseous productsfrom the rod is sufficiently slow to minimize microexplosions of gaseousproducts that might adversely affect the structural integrity of therod. Preferably, the rate of temperature increase should be in the rangeof about 1° C./min to about 20° C./min and more preferably, in the rangeof about 5° C./min to about 10° C./min. The temperature is increased atthis rate until the maximum temperature is reached and the carbonaceousbinders are decomposed. Preferably, the maximum temperature is betweenabout 650° C. to about 900° C., and more preferably in the range ofabout 675° C. to about 825° C.

The maximum temperature and the length of time the rods remain at themaximum temperature determines the strength of the rod and its chemicalcomposition. The strength of the rod should be sufficient to withstandhigh speed manufacturing processes, although the strength may beadjusted to match a particular application.

Reduction of the metal species occurs during the heating process bycontact with a reducing gas. During heating, the carbonaceous binderdecomposes to yield, inter alia, carbon monoxide, a reducing gas.Furthermore, when activated carbon is used as the carbon source, itreacts with the metal species to generate carbon monoxide and apartially reduced metal species.* With each reduction of the metalspecies, the CO is oxidized to CO₂. Rather than rely totally on carbonand the evolved CO to reduce the metal species, a reducing agent such ashydrogen gas may be added to the atmosphere of the heating device.Preferably CO and CO₂ may be be added directly to the heating deviceatmosphere.

The ratio of CO to CO₂ can be manipulated to control end productdistribution. The presence of a ratio of CO to CO₂ of between about 0.16and about 6 and more preferably between about 0.3 and about 2.5 has beenfound to increase the strength of the rod and improve the lightability(the time required to ignite the final heat source) of the final metalnitride-comprising product. Furthermore, the phase homogeneity of themetal products depends upon the CO/CO₂ ratio. The preferred CO/CO₂ ratiofor a particular application may be found by using the method describedby H. L. Fairbanks, Industrial Heating, 52, pp. 24-26 (1984).

The extent of reduction of the metal species during the binder burnoutstage is controlled by the baking temperature and the duration ofbaking. For example, the reduction of metal species may be complete oncethe maximum temperature is reached. If not, the maximum temperatureshould be maintained until the metal species is sufficiently reduced. Atthe termination of the binder burnout stage, the preferred metal productis substantially a mixture of a metal oxide of low valency*, and thefully reduced metal.

Upon completion of the binder burnout stage, the rods are cooled andthen contacted with a nitriding material to produce metal nitride. Thedegree of nitridation and the phase of metal nitride produced willaffect the lightability of the heat source. A preferred nitridingmaterial is ammonia. While not wishing to be bound by theory, it isbelieved that the ammonia reacts preferentially with the fully reducedmetal component, leaving the low valency metal oxide substantiallyunreacted. Preferably the nitriding temperature range is between about400° C. to about 600° C., and more preferably, between about 450° C. toabout 550° C. Preferably, the duration of the nitriding step can rangeup to about 150 minutes, and preferably between about 30 and about 120minutes.

The metal nitride produced by the above method may contain localizedpyrophoric sites having increased reactivity, which must be passivated.Passivation involves the controlled exposure of the heat source to anoxidant. Preferred oxidants include dilute oxygen, or, more preferably,dilute air. While not wishing to be bound by theory, it is believed thata low concentration of oxidant will eliminate pyrophoric sites whilepreventing the uncontrolled combustion of the heat source.

As stated above, variations in the dimensions of the rod will occurduring baking. Generally, between about 10% to about 20% change involume will occur as a result of the binder burnout. This change involume may cause warping or bending. The rod may also sufferinconsistencies in diameter. Following nitridation, therefore, the rodmay be tooled, or ground to the dimensions described above. The rod isthen cut into the shortened segments of between about 8 mm to about 20mm, preferably between about 10 mm to about 14 mm.

The rod produced by this method comprises (1) between about 30% andabout 40% carbon; (2) between about 40% and 50% metal nitride; and (3)between about 10% and about 20% low valency metal oxide. The rod mayadditionally contain trace amounts of a high valency metal oxide. Thechemical composition of the rods is between (1) about 30% and about 40%carbon; (2) about 0.46% and about 0.62% hydrogen; (3) about 2.08% andabout 4.3% nitrogen; (4) about 60% and 70% metal; (5) about 3% and about6% oxygen.

The metal nitride component of the heat source has an ignitiontemperature that is sufficiently low to allow for ignition under theconditions for lighting a conventional cigarette. Upon combustion, thecarbon component provides additional heat so that the heat source doesnot prematurely self-extinguish. Combustion of the low valency metaloxide component provides heat upon combustion and acts as a catalyst topromote the oxidation of CO to CO₂.

The ignition temperature of the heat source is preferably in the rangeof between about 175° C. and about 450° C., and, more preferably betweenabout 190° C. and about 400° C. Upon ignition, the heat sources reach amaximum temperature preferably between about 600° C. and about 950° C.and, more preferably, between about 650° C. and about 850° C. Themaximum temperature will depend in part upon the smoking conditions andany materials in contact with the heat source which affect theavailability of oxygen. Thus, metal nitrides are substantially easier tolight than conventional carbonaceous heat sources and less likely toself-extinguish, but at the same time can be made to smolder at lowertemperatures, thereby minimizing the risk of fire.

The heat sources made by the method of this invention are stable under abroad range of relative humidity conditions and aging times. Forexample, aging of heat source up to three months under a variety ofrelative humidity conditions ranging from about 0% relative humidity toabout 100% relative humidity should have virtually no effect on thecombustion products. Furthermore, the heat sources undergo virtually nochange in dimensions upon aging.

Any metal nitride capable of combusting without generation of nitrogenoxides may be used in the heat sources of this invention. Preferably,the metal nitride is zirconium nitride, and, more preferably, ironnitride having the formula Fe_(x) N, where X is between 2 and 4inclusive. Mixtures of metal nitrides may also be used.

EXAMPLE 1

710 g Fe₂ O₃, 250 g activated carbon, 175 g water, and 37.5 g potassiumcitrate were combined with a binder made from 200 g flour, 103 g sugarand 22 g corn oil. The Fe₂ O₃ /activated carbon/binder mixture was thenextruded to form green rods 30 cm in length and 5.05 mm in diameter witha single star air flow passageway. The green rods were placed ongraphite sheets and stacked in a fixed-bed reactor at room temperature.Argon with a flow rate of 1 lit/min was used as a carrier gas to purgethe gaseous content of reactor. The rods were heated at a rate of 1°C./min until a temperature of 100° C. was reached; the temperature wasthen increased at a rate of 5° C./min until a temperature of 750° C. wasreached. The temperature was maintained at 750° C. for 60 minutes. Thereactor was then cooled down at a rate of 10° C./min to a temperature of450° C. The reactor was purged with dry ammonia and the rods nitridedfor 60 minutes at 450° C. The reactor was then cooled to ambienttemperature and the reactor purged with argon. The rods were thenpassivated by introducing O₂ until a concentration of 5% O₂ was reached.The iron nitride-comprising rod was then cut with a saw into segments of14 mm to form the heat sources.

EXAMPLE 2

The heat source was produced using conditions identical to those used inExample 1 except that during the heating stage the argon was replaced bya an atmosphere of 75% argon, 25% of a 2:1 mixture of CO/CO₂.

EXAMPLE 3

Chemical composition of an iron nitride heat source made by the methoddescribed in Example 1:

Carbon: 32.6%

Fe_(x) N : 48.8%

FeO : 18.5%

EXAMPLE 4

Combustion of heat sources made by the method described in Example 1under FTC conditions were performed and the following analyses obtained:

Ignition temperature : 240° C.

Total CO evolved : 0.38 mg/heat source

Total CO₂ evolved : 14.82 mg/heat source

Total NO evolved : 0.01 mg/heat source

The heat sources and processes described above employ a conventionalheating or baking device (e.g., a muffle furnace or a sagger) to supplyheat to the green rods. In an alternate embodiment of the presentinvention, the heating or baking device can be a microwave or radiofrequency (RF) baking device. According to this aspect of the presentinvention, the green rods can be placed in a microwave or RF heatingdevice to provide the necessary heat for solvent vaporization, binderburn out or nitradation of the metal nitride heat source.

The microwave/RF heating step described herein has some advantages overthe conventional heating step. In particular, because the heatingprocess is concentrated within the green rod itself, rather than thefurnace in which the green rod is placed, the heating efficiency isincreased. Furthermore, such internal healing results in thermalgradients that are the reverse of those observed in conventionalheating. This can help in efficient removal of volatile constituents(e.g., solvent vaporiztion and binder burnout) and can reduce thethermal stresses that sometimes causes cracking during thermalprocessing. For example, heat sources that are made with a conventionalheating step of over ten hours can be made with a microwave/RF heatingstep of only a few hours. Such reduced processing times cansignificantly reduce the cost of the heat source.

Although the microwave/RF heating step described above refers to the useof "internal" heating of the green rod (commonly referred to as "primaryheating"), it will be apparent that the microwave/RF heating step canalso be used to "externally" heat the green rod by placing the green rodon a susceptor that is capable of being heated by the microwave/RFenergy. In accordance with this aspect of the present invention, whenthe susceptor is heated, it transfers such heat to an adjacent green rodby way of conduction, convection, radiation, or combinations thereof.Such heating is commonly referred to as "secondary heating." It willalso be apparent that one could simultaneously combine an internalheating process with an external heating process, and, if desired, witha conventional heating process.

The microwave/RF heating step described herein employs electromagneticradiation to generate heat either internal to the heat source orinternal to a susceptor that transfers such heat to the heat source. Thefrequency of such electromagnetic radiation is chosen to provide for theefficient generation of such heat. For example, frequencies in the rangefrom radio frequencies to microwave frequencies are believed to beacceptable depending upon the particular materials being heated. Acommon yfrequency used in microwave processing of materials is 2.45 GHz.Of course, other frequencies can be used as well.

It will be apparent to those of skill in the art that if a microwave/RFheating step is used to supply heat to the green rod mixture to form themetal nitride heat source, the times and temperatures for solventvaporization, binder burnout and nitradation should be accordinglyadjusted. The times and temperatures can be readily determinedexperimentally, as is the case for conventional heating, and will ofcourse depend upon the particular microwave/RF heating device employed.

Although the microwave/RF heating step described above has beendiscussed with particular reference to a metal nitride heat source, itwill be apparent that it is equally applicable to other types of heatsources as well.

For example, above-incorporated Nystrom et al. U.S. Pat. No. 5,076,296(PM-1319) describes a carbonaceous heat source formed from charcoalparicles. According to the present invention, the carbonaceous heatsource described in that patent can be made using the microwave/RFheating step described herein. In particular, after mixing the charcoalparticules with a desired additive, and extruding or molding the mixtureinto a desired shape, the extruded material can be baked in themicrowave/RF heating device described herein.

Additionally, copending, commonly-assigned U.S. patent application Ser.No. 07/732,619 (PM-1353), filed Jul. 19, 1991, which is herebyincorporated by reference in its entirety, describes a heat sourceformed from a carbon source and a metal oxide. According to the presentinvention, the heat source described in that application can also bemade using the microwave/RF heating step described herein. Inparticular, after mixing the metal oxide with the carbon source, themixture can be placed in a microwave/RF heating device to supply thenecessary heat to form the metal species described in that application.

Furthermore, copending, commonly-assigned U.S. patent application Ser.No. 07/639,241 (PM-1336), filed Jan. 1, 1991, which is herebyincorporated by reference in its entirety, describes a method for makinga metal carbide heat source. That method includes the step of supplyingheat to a mixture of a metal species and a carbon source. According tothe present invention, that step can include the step of supplyingmicrowave/RF energy to heat the mixture.

Similarly, Deevi et al. U.S. Pat. No. 5,146,934 (PM-1360) and copending,commonly-assigned U.S. patent application Ser. No. 07/556,732 (PM-1347),filed Jul. 20, 1990, which are both hereby incorporated by reference intheir entireties, describe other types of heat sources for use insmoking articles. Those heat sources are also made using a heating stepfor supplying heat to a mixture. In accordance with the presentinvention, the heating step can include the step of supplyingmicrowave/RF energy to heat the mixture.

Thus, this invention provides a process for making a heat sourcecomprising a substantial content of metal nitrides; metal oxides; andcarbon, that forms virtually no carbon monoxide gas upon combustion andhas a significantly lower ignition temperature than conventionalcarbonaceous heat sources, while at the same time maximizes heattransfer to the flavor bed. One skilled in the art will appreciate thatthe present invention can be practiced by other than the describedembodiments, which are presented herein for the purpose of illustrationand not of limitation, and that the present invention is limited only bythe claims which follow.

We claim:
 1. A method of making metal nitride, comprising the stepsof:a) mixing a metal species and a carbon source to form a mixture; b)supplying heat to the mixture, wherein the heat is provided byelectromagnetic radiation having a frequency in the range from radiofrequency to microwave frequency; and c) contacting the mixture with anitriding material to form metal nitride.
 2. A method of making a heatsource comprising metal nitride, comprising the steps of:a) mixing ametal species and a carbon source to form a mixture; b) combining themixture from step a) with a binder; c) forming the mixture from step b)into a shape; d) supplying heat to the mixture, wherein the heat isprovided by electromagnetic radiation having a frequency in the rangefrom radio frequencies to microwave frequencies; and e) contacting themixture with a nitriding material to form metal nitride.
 3. The methodof claim 1 or 2, wherein the metal species is an iron species.
 4. Themethod of claim 3, wherein the iron species is selected from the groupconsisting of Fe₂ O₃, FeOOH, Fe₃ O₄, FeO, or Fe⁰.
 5. The method of claim1 or 2, wherein the carbon source is selected from the group consistingof colloidal graphite and activated carbon.
 6. The method of claim 1 or2, wherein the heat is supplied by a microwave oven.
 7. The method ofclaim 1 or 2, wherein the nitriding material is a nitrogenous gas. 8.The method of claim 2, wherein the binder comprises a carbonaceousmaterial.
 9. The method of claim 2, wherein the carbonaceous materialcomprises a mixture comprising flour, sugar, and a vegetable oil. 10.The method of claim 2, wherein in step c) the mixture is formed into acylindrical rod.
 11. The method of claim 1 or 2, wherein the metalspecies and carbon source are combined in a polar solvent.
 12. Themethod of claim 1 or 2, wherein the metal species is in particulate formhaving a particle size of up to about 300 microns.
 13. The method ofclaim 1 or 2, wherein the metal species is in particulate form having aparticle size of between about submicron and about 20 microns.
 14. Themethod of claim 1 or 2, wherein the metal species has a surface area ofbetween about 0.2 m² /g and about 300 m² /g.
 15. The method of claim 1or 2, wherein the metal species has a surface area of between about 1 m²/g and about 150 m² /g.
 16. The method of claim 1 or 2, wherein thecarbon source is in particulate form having a particle size of up toabout 300 microns.
 17. The method of claim 1 or 2, wherein the carbonsource is in particulate form having a particle size of between aboutsubmicron and about 40 microns.
 18. The method of claim 1 or 2, whereinthe carbon source has a surface area of between about 0.5 m² /g andabout 2000 m² /g.
 19. The method of claim 1 or 2, when in the carbonsource has a surface area of between about 100 and about 600 m² /g. 20.A method of producing a composite carbon and metal species heat sourcecomprising:a) mixing a metal oxide and a carbon source; b) supplyingheat to the mixture in an environment which allows the formation of ametal species, wherein the heat is provided by electromagnetic radiationhaving a frequency in the range from radio frequency to microwavefrequency; and c) cooling.
 21. A method for producing a composite heatsource, comprising:a) mixing a metal oxide and a carbon source; b)combining the mixture from step a) with a binder; c) forming the mixturefrom step b) into a shape; d) supplying heat to the mixture in anenvironment which allows for the reduction of the metal species, whereinthe heat is provided by electromagnetic radiation having a frequency inthe range from radio frequency to microwave frequency; and e) coolingthe shape foraged in step c).
 22. A method for producing a compositeheat source, comprising:a) mixing a metal oxide, metal and a carbonsource; b) combining the mixture from step a) with a binder c) formingthe mixture from step b) into a shape; d) supplying heat to the mixturein an environment which allows for the reduction of the metal species,wherein the heat is provided by electromagnetic radiation having afrequency in the range from radio frequency to microwave frequency; ande) cooling the shape formed in step c).
 23. The method of claim 20, 21or 22, wherein the metal oxide is iron oxide.
 24. The method of claim 23wherein the iron oxide is selected from the group consisting of Fe₂ O₃,Fe₃ O₄, and FeO.
 25. The method of claim 20, 21 or 22, wherein thecarbon source is selected from the group consisting of colloidalgraphite and activated carbon.
 26. The method of claim 20, 21 or 22,wherein the heat is supplied by a microwave oven.
 27. The method ofclaim 21 or 22, wherein the binder comprises carbonaceous materials. 28.A process for making a heat source for a smoking article comprising thesteps of:(a) mixing charcoal particles of the desired size with desiredadditives for a set period of time; (b) extruding or molding saidcharcoal and additives into the desired shape; and (c) heating saidextruded material, wherein the heat is provided by electromagneticradiation having a frequency in the range from radio frequency tomicrowave frequency.
 29. The process of claim 28, wherein the heat issupplied by a microwave oven.
 30. A method of producing metal carbide,comprising the steps of:a. mixing a metal species and a carbon source,wherein the carbon source is selected from the group consisting ofcolloidal graphite, activated carbon and activated charcoal; b.supplying heat to the mixture, wherein the heat is provided byelectromagnetic radiation having a frequency in the range from radiofrequency to microwave frequency; and c. contacting the mixture with areducing/carbidizing atmosphere.
 31. A method of producing a heat sourcecomprising a metal carbide, comprising the steps of:a. mixing a metalspecies and a carbon source; b. combining the mixture from step a) witha binder; c. forming the mixture from step b) into a shape; d. supplyingheat to the mixture, wherein the heat is provided by electromagneticradiation having a frequency in the range from radio frequency tomicrowave frequency; and e. contacting the mixture with areducing/carbidizing atmosphere.
 32. The method of claims 30 or 31,wherein the metal species is an iron species.
 33. The method of claim32, wherein the iron species is selected from the group consisting ofiron oxyhydroxide, Fe₂ O₃, Fe₃ O₄, FeO and mixtures thereof.
 34. Themethod of claim 30 or 31, wherein the carbon source is selected from thegroup consisting of colloidal graphite and activated carbon.
 35. Themethod of claim 30 or 31, wherein the heat is supplied by a microwaveoven.
 36. A method of producing a heat source comprising the steps of:a.mixing a carbon component, a catalytic precursor, and a binder, whereinthe catalytic precursor is a metal species which upon combustion of theheat source forms a catalyst for converting carbon monoxide producedduring combustion of the heat source to a benign substance; b. formingthe mixture into a shape; and d. supplying heat to the mixture, whereinthe heat is provided by electromagnetic radiation having a frequency inthe range from radio frequency to microwave frequency.
 37. The method ofclaim 36, wherein the metal species is an iron species.
 38. The methodof claim 36, wherein the metal species is Fe₅ C₂.
 39. The method ofclaim 36, wherein the carbon component is selected from the groupconsisting of colloidal graphite and activated carbon.
 40. The method ofclaim 36, wherein the heat is supplied by a microwave oven.